1. Field of the Invention
The present invention relates to an image forming method for forming an image on an organic or inorganic semiconductor substrate with an electrochemical reaction, and to an image forming apparatus suited for use in the image forming method.
2. Description of the Related Art
Methods that are currently utilized in a printer or the like for the purpose of transferring an image from an electric signal or optical signal to a recording medium such as paper include, for example, dot-impacting, thermal transfer, thermal sublimation, ink jet, and electrophotographic methods of laser printers. These methods are roughly divided into three groups.
The methods that are included in the first group are those based on, for example, dot-impacting, thermal transfer, and thermal sublimation. According to such methods, a sheet, which is in the form of an ink ribbon or a donor film and which contains a dispersed dye, is superposed on paper or the like and a dye is transferred to the paper by an application of a mechanical impact or heat. The methods that are included in the second group are those based on, for example, an ink jet method, in which an ink is directly transferred to paper from a head. The methods that are included in the third group are those based on electrophotographic methods of laser printers and the like in which toner particles are made to adhere to an electrostatic latent image created by laser spots on a photoreceptor and then the toner particles are transferred to paper to form an image.
Problems associated with these methods are as follows. In the case of the methods of the first group which require pressing and heating, problems are difficulty in high-speed operation, poor energy efficiency, and high running costs. In the case of the methods of the second group, a problem is difficulty in high-speed operation, because it is difficult to electrically control ink dots and to form a head corresponding to the width of the paper. Another problem is that the minimal image unit is restricted by the size and interval of the head. In the case of the methods of the third group, which are electrophotographic methods, problems include the fact that a high voltage is necessary to form an electrostatic latent image and to adhere/transfer toner particles and a large amount of electricity is consumed, generating ozone and nitrogen oxides.
In conclusion, none of the image forming methods adopted in printers and the like at the present time is a general-purpose method leading to a high-quality image, a relatively high speed, a low level of running costs, energy saving, resource saving, and advantages for both the environment and for users. This is because an image forming method in which a dye is directly controlled at a molecular level to form an image has not been established.
After studying the principles of these electrodeposition technologies, the present inventors have found that some water-soluble dye molecules significantly change their solubility in water depending on their states, i.e., an oxidized state, a neutral state, or a reduced state.
Examples of compounds having such a property are as follows. Rose Bengal end eosine, which are each a fluorescein dye, are in a reduced state and are soluble in water at a pH value of 4 or more. But, at a pH value of less than 4, these dyes are oxidized to a neutral state, depositing and/or precipitating in water. Further, it is generally known that the solubility of a dye having a carboxyl group significantly changes depending on the hydrogen ion concentration (pH) of the solution even if a structural change of the dye does not take place. Specifically, an ink-jet dye whose water resistance has been improved dissolves in water at a pH value of 6 or more but precipitates at a pH value of less than 6. If electrodes are immersed in a solution comprising any of these dyes dissolved in pure water and voltage is applied, an electrodeposited film composed of the dye molecules is formed on the anode. Likewise, a water-soluble acrylic resin, which is a polymer having a carboxyl group, dissolves in water at a pH value of 6 or more but precipitates at a pH value of less than 6. If electrodes are immersed in a liquid comprising a pigment dispersed in a solution of the polymer and a voltage is applied, the pigment. and the polymer are deposited on the anode to thus form an electrodeposited film comprising a mixture of the pigment and the polymer. These electrodeposited films can be redissolved in a solution either by applying a reverse voltage or by immersing the films in a solution having a pH value in the range of 10 to 12. Meanwhile, an oxazine-based basic dye, i.e., Cathilon Pure Blue 5GH (C.I. Basic Blue 3), and a thiazine-based basic dye, i.e., Methylene Blue (C.I. Basic Blue 9)H, which are each a quinoneimine dye, are in an oxidized state at a pH value of 10 or less and are colored. But, at a pH value greater than that, these dyes are brought to a reduced state, becoming insoluble and depositing. If electrodes are immersed in a solution comprising any of these dyes dissolved in pure water and a voltage is applied, an electrodeposited film composed of the dye molecules is formed on the cathode. These electrodeposited films are redissolved in a solution either by applying a reverse voltage or by immersing the films in a solution having a pH value of 8 or less.
According to traditional electrodeposition technology, the voltage required for the formation of an electrodeposited film is as high as about 7V. If such a high voltage is applied, an image cannot be formed because the Schottky barrier between the semiconductor and the electrolyte solution is destroyed.
Although a method is proposed in which a dye is used in doping/dedoping of an electroconductive polymer so that an image is formed with light, an electrodeposited film can be formed with a dye alone without the use of an electroconductive polymer. However, the voltage required for forming the electrodeposited film with the dye alone is larger than the voltage required for the formation of the electrodeposited film in the presence an electroconductive polymer. Meanwhile, the photovoltaic force of a general-purpose Si-based photo-semiconductor is about 0.6V, which by itself is insufficient for image formation. Accordingly, although measures are contrived such as application of a bias voltage to bolster the voltage, a voltage larger than a certain value (i.e., a voltage that depends on the band gap of the semiconductor to be used) destroys the Schottky barrier, which is required for the formation of the photovoltaic force and which is present between the semiconductor and the electrolyte solution. Accordingly, the bias voltage to be applied should not exceed a limit. Because of this, image forming methods effected in an aqueous solution by using photovoltaic force have been limited to methods that use a photopolymerization reaction of an electroconductive polymer capable of undergoing an oxidation-reduction reaction at a voltage of 1.0V or less such as polypyrrole. In Japanese Patent Application Laid-Open (JP-A) No. 5-119,209 ("A method for manufacturing a color filter and an electrodeposition substrate for manufacturing the color filter") and JP-A No. 5-157,905 ("A method for manufacturing a color filter"), which are well-known in this field, the electrodeposition voltage is as high as 20 to 80V and the electrodepositable material utilizes an oxidationreduction reaction of a polymer. Accordingly, the voltage that is required for electrodeposition of polymers generally known as a material for electrodeposition coating, is 10V or more. Therefore, despite attempts such as attempt to utilize for image formation the photoconductive property of ZnO.sub.2, which is used in electrophotography, a practical electrodepositable material that can be easily handled and can be used in an aqueous liquid has not yet been found.